The present invention is in a process for the catalytic preparation of alkali alcoholates from alkali amalgams and alcohols.
Alkali alcoholates, especially those whose alcohol component contains up to 4 carbon atoms, are valuable chemicals. They are used, for example, as catalysts in the synthesis of many organic compounds. The alcoholates of sodium and potassium have acquired particular practical importance. Several methods are known for the preparation of alkali alcoholates (F. A. Dickes, Ber. Dtsch. Chem. Ges. 63, 2753 [1930]). Thus, solutions of alkali hydroxides in an alcohol contain alkali alcoholates in equilibrium. By removing the water that is in this equilibrium, by distillation for example, pure alkali alcoholates are obtained. Especially in the case of low-boiling alcohols, however, a great deal of energy is needed for this kind of equilibrium alteration.
Pure alkali alcoholates are arrived at directly by dissolving an alkali metal in the corresponding alcohol. Sodium and potassium react vigorously with low aliphatic alcohols, such as methanol and ethanol, with the evolution of hydrogen. Higher alcohols, such as the propanols and butanols, are reacted with alkali metals preferably above the melting point of the latter, with stirring and under pressure if required. The method of direct preparation of the alkali alcoholates from metal and alcohol scarcely qualifies as a commercial process, because the alkali metals required as the starting materials are too expensive.
It is more economical to use as the alkali source the liquid alkali amalgam that is produced in the electrolysis of chlorinated alkali by the mercury method.
The reaction of alkali amalgam with alcohols and the use of catalysts for this reaction are known. R. B. MacMullin, in Chemical Engineering Progress, September 1950, p. 440, mentions, among other things, the reaction of alkali amalgam with methanol in a reactor which contains graphite as catalyst. In U.S. Pat. Nos. 2,336,045 and 2,761,880, nonamalgamating substances, such as iron, graphite or mixtures thereof, are proposed as catalysts. In U.S. Pat. No. 2,069,403, metal meshes made of heavy metal alloys are described as catalysts.
The problem was to find a process that satisfies the following requirements.
The reaction between the amalgam and the alcohol should take place at the highest possible rate in order to achieve high volume-time yields in a technical reactor. Furthermore, after this reaction the concentration of the alkali metal in the amalgam should be as low as possible, since the residual content of alkali metal has to be reacted with water in a second reaction to form alkali hydroxide solution, which is not a target product of the present process. This reaction of the amalgam with water is necessary in order to completely react the alkali metal contained in the amalgam. Otherwise, when the mercury is recycled into the electrolysis cell the alkali metal concentration would become too high and the amalgam would become solid. The process should be performable in a continuous manner. In a technical decomposer with a bed of a known catalyst of 1 m height, and 0.2 to 0.35 wt.-% of sodium or potassium in the mercury at the inlet, the residual content of alkali metal in the amalgam at the outlet will be between 50 and 70% of the inlet concentration.